I would say the next step would be to plug it into an OBDII diagnostic tool to see if there are any fault codes and if the modules are all talking to each other. I'm not sure of the decent cheaper options to do this on Nissans e.g. elm327 adapter and some software. I have used NDSII lite (v1.53) with an existing bluetooth elm327 adapter on my families k12's. Although I've not used extensively it can show certain PIDs, fault codes and appears to have some self diag feature although i found it a bit flaky when trying to connect to my bluetooth elm327. Any decent auto electrician should have much better equipment to diagnose this if you went the garage route.
Of course you could also try disconnecting the battery for half an hour to see if that fixes it by resetting the modules (although you might have effectively done that when cleaning up the chassis ground connections.)
Edit : Also try the K12 Owners UK group on facebook someone on there might have experienced something similar.